Post-SB 343: why your 2023 California child-support calculator is wrong

If you ran your California child-support estimate on a calculator built before September 2024, the number it gave you is wrong. Not “slightly off.” Wrong by the standards the statute itself sets — by tens of percent in some income bands, by hundreds of dollars per month in others. The K-factor table that the calculator was using does not exist anymore.

This is not subtle and it is not a technicality. Senate Bill 343 (Stats. 2023, Ch. 213, Skinner) repealed the entire pre-2024 §4055 and added a new §4055 in its place, operative September 1, 2024. The new K-table has different income bands and different multipliers. The Low-Income Adjustment threshold is structurally different — it doesn’t index against a dollar number anymore; it indexes against the California minimum wage. And published appellate guidance on the new K-table is essentially nonexistent — the statute is too new — which means anyone running unupdated math has nothing to fall back on.

This post is the side-by-side. What changed, why it changed the answer, and where to check whether the calculator you’re about to rely on is current.

What §4055 actually does

The §4055 guideline formula computes monthly child support as:

CS = K × [HN − (H% × TN)] × multiChildMultiplier

• TN is the parents’ combined net disposable income per month.
• HN is the higher-earning parent’s net disposable income.
• H% is the higher-earner’s custody timeshare as a decimal (0–1).
• K is the percentage of combined income allocated to support — built from a timeshare multiplier and an income fraction.
• multiChildMultiplier scales the result up for two or more kids.

Almost every public discussion of the formula focuses on K. There is a reason for that: K is the one piece of the formula that the legislature controls directly through the §4055(b)(3) income-band table, and changing K is how SB 343 actually moved the support number. The bracketed [HN − H% × TN] term is just income-and-custody arithmetic; the multi-child multiplier is small. K is where the rule lives.

The old K-table (operative through August 31, 2024)

Before SB 343, the §4055(b)(3) table had four income bands:

Total Net Disposable Income Per Month	K (income fraction)
$0 – $800	0.20 + TN/16,000
$801 – $6,666	0.25
$6,667 – $10,000	0.10 + 1,000/TN
Over $10,000	0.12 + 800/TN

That table — verbatim, from the SB 343 chaptered bill text, Section 2 (which shows the old text being repealed) — is what California family courts ran from the mid-1990s through 2024. It is also what almost every calculator on the open internet still encodes if it hasn’t been updated.

The new K-table (operative September 1, 2024)

SB 343 replaced that table with five bands, and re-shaped the very-low-income end specifically:

Total Net Disposable Income Per Month	K (income fraction)
$0 – $2,900	0.165 + TN/82,857
$2,901 – $5,000	0.131 + TN/42,149
$5,001 – $10,000	0.250
$10,001 – $15,000	0.10 + 1,499/TN
Over $15,000	0.12 + 1,200/TN

Verbatim from the current §4055 at leginfo.legislature.ca.gov. The chaptering language at the bottom of that section reads, verbatim:

Repealed (in Sec. 2) and added by Stats. 2023, Ch. 213, Sec. 3. (SB 343) Effective January 1, 2024. Operative September 1, 2024, by its own provisions.

The structural change is in the lower income bands. The old table had a flat 0.25 plateau that started at just $801 of monthly combined net. The new table replaces that with two graduated bands — a $0–$2,900 band built around 0.165 + TN/82,857, and a $2,901–$5,000 transition band built around 0.131 + TN/42,149. The flat 0.25 doesn’t kick in until $5,001 now.

At low TN — under roughly $3,000 of combined monthly net — that is a real reduction. At the $1,000 example the legislature itself uses in §4055(b)(3) (“K = 0.21” post-SB-343 vs. “K = 0.30” under the old rule for the same H% = 20%), K drops by about thirty percent.

The buried lede: the low-income presumption is structurally different

The K-table change gets all the press. The bigger change is two paragraphs further down, in §4055(b)(7) — the Low-Income Adjustment.

The old §4055(b)(7) read:

In all cases in which the net disposable income per month of the obligor is less than one thousand five hundred dollars ($1,500), adjusted annually for cost-of-living increases, there is a rebuttable presumption that the obligor is entitled to a low-income adjustment…

A fixed $1,500/month net threshold, indexed by the Judicial Council against the California CPI. To qualify, the obligor’s net monthly income had to fall below ~$1,500 (CPI-adjusted). In practice that meant only obligors near or below the federal poverty line.

The new §4055(b)(7) reads:

In all cases in which the net disposable income per month of the obligor is less than the amount of monthly gross income earned from full-time minimum wage, established by Section 1182.12 of the Labor Code, at 40 hours per week, 52 weeks per year, there is a rebuttable presumption that the obligor is entitled to a low-income adjustment…

Three things changed at once:

1. The comparison basis flipped from “obligor’s net vs. $1,500 net” to “obligor’s net vs. full-time-minimum-wage gross.” Comparing a net number to a gross number is structurally easier for the net to fall below.
2. The threshold size roughly doubled. At California’s 2026 minimum wage, $16.90/hour × 40 × 52 ÷ 12 ≈ $2,929/month gross. The old fixed floor was $1,500/month net.
3. The indexing mechanism switched from “Judicial Council annual COLA based on CA CPI” to “automatically tracks the minimum wage.” Every time the legislature raises the minimum wage, the LIA threshold rises with it — no rule-making step in between.

The LIA reduction itself is rebuttable — the trial court can find that the presumption doesn’t apply if the §4053 principles (the policy goals at the top of the statute) don’t support reducing the obligor’s payment. But the presumption attaches to a much larger universe of obligors than it did under the old rule.

What the change looks like in dollars

Concrete is better than abstract. A single fact pattern, run through both the old and new rules — the numbers below come straight out of the CleanCalc engine for the post-SB-343 case, and from the old statutory math for the pre-SB-343 case.

Inputs:

• Obligor: $2,000/month net disposable income, 20% custody timeshare.
• Other parent: $500/month net disposable income, 80% custody (primary).
• One child, no §4062 add-ons, no §4059 deductions beyond what’s already in NDI.
• Combined monthly net (TN) = $2,500.

Pre-SB-343 (operative through August 31, 2024):

• Income fraction = 0.25 (flat $801–$6,666 band).
• Timeshare multiplier = 1 + 0.20 = 1.20 (since H% ≤ 0.5).
• K = 1.20 × 0.25 = 0.30.
• CS = 0.30 × ($2,000 − 0.20 × $2,500) = 0.30 × $1,500 = $450/month.
• LIA: obligor’s net ($2,000) is greater than the old $1,500 floor → no LIA presumption → final answer $450/month.

Post-SB-343 (operative September 1, 2024):

• Income fraction = 0.165 + (2,500 / 82,857) ≈ 0.195 (the new $0–$2,900 band).
• Timeshare multiplier = 1.20 (unchanged).
• K = 1.20 × 0.195 ≈ 0.234.
• CS before LIA = 0.234 × $1,500 ≈ $351/month.
• LIA: obligor’s net ($2,000) is less than the new minimum-wage-gross threshold (~$2,929) → LIA presumption attaches → engine computes a presumed reduction of about $111/month.
• Final CS = $351 − $111 ≈ $240/month.

That’s a drop from $450 to $240 per month at this fact pattern — a $210/month difference, or about $2,520/year. Roughly half of that is the K-table change; roughly half is the new LIA presumption catching an obligor the old rule didn’t.

Different fact patterns produce different deltas. A high-income case where TN > $5,000 lands back in the same 0.25 plateau both pre and post, and the K-table change largely vanishes — the LIA threshold change still matters for the marginal obligor. The cases where SB 343 actually moves the needle are concentrated in the low- and lower-middle-income bands.

What about case law?

Almost nothing. SB 343’s new §4055 has been operative for roughly twenty-one months as of the date of this post. Family-court appellate decisions move slowly. We are aware of one published case that touches §4055 in the relevant period — Mamer v. Weingarten (2025) 108 Cal.App.5th 169 — and it does not interpret the new K-table; it cites §§4055 and 4058 in passing for the proposition that a family court considering equitable IVF-cost reimbursement should weigh the parents’ incomes. Not a K-factor case.

There are unpublished decisions that run the new K-table on real fact patterns, but California Rules of Court 8.1115(a) prohibits citing them as authority. They are useful as a sanity check that trial courts are running the new math, not as legal precedent.

For the holdings that describe §4055’s algebraic structure — that it is “a rigid algebraic formula” producing a “presumptively correct” result that the court may deviate from only on §4057(b) findings — the still-controlling authorities are pre-SB-343: In re Marriage of Cryer (2011) 198 Cal.App.4th 1039 and In re Marriage of Hall (2000) 81 Cal.App.4th 313. The algebraic-rigidity doctrine survived the bill. The K-table values did not.

How to tell if the calculator you’re about to use is current

Three checks, in order of how quickly they reveal the answer:

1. Look for the operative date. A current calculator will say something like “post-SB-343,” “effective September 1, 2024,” or “2024 K-factor table.” If the page makes no mention of SB 343 or the September 2024 operative date, the math is probably old.
2. Check the LIA threshold. Type in an obligor net income of $2,000 and look at whether the LIA presumption attaches. Under the old rule it should not (because $2,000 > $1,500). Under the new rule it should (because $2,000 < $2,929 at 2026 minimum wage). If the calculator silently skips the LIA presumption at $2,000, it’s running the old rule.
3. Look at K at $1,000 TN. The legislature’s own example pegs the post-SB-343 K at 0.21 for H% = 0.20 at TN = $1,000. The old rule produced K = 0.30 at the same inputs. If you can see K (or compute it backward from the displayed support), you can tell which table is in use.

Our /california/support/calculator runs the post-SB-343 §4055 — verbatim — with the new K-table and the new LIA threshold, and shows the work for both. If our number doesn’t match a court order or an attorney calculation, you should be able to see exactly which band the K came from and which threshold the LIA used. That is the whole point.

If you’re a self-employed parent, the §4058 income definition is the other half of the puzzle — the K-table operates on net disposable income, and how net disposable income gets computed from a 1099 or Schedule C is its own can of worms.

What this post is not

This is a walk-through of a statute change and the math it produced. It is not legal advice. California family judges retain wide discretion under §4057 to deviate from any guideline result for stated reasons. Cases with imputation under §4058(b), contested business valuations, or §4070-4073 hardship deductions benefit from a licensed California family-law attorney’s review — particularly before any court filing. If your case is contested or your income picture is non-trivial, talk to one.

The statute text quoted above was current as of the statute_as_of date in this post’s frontmatter; we re-verify all citations quarterly. The page footer’s disclaimer applies to every section above.